The present invention relates to a method for growing a ZnO based oxide semiconductor layer, and a method for manufacturing a semiconductor light emitting device using the same. More specifically, the present invention relates to a method for growing a ZnO based oxide semiconductor layer in which the ZnO based oxide layer grown has the excellent flatness and has little crystal defects.
Recently, it has become possible to obtain a light emitting diode (hereinafter, referred to as LED) in blue-based color (which means the wavelength range from ultraviolet to yellow, herein after, indicating the same meaning) used for a full-color display and a signal light, and a blue-based color laser for use in next-generation high-definition DVD light source which continuously emits light at room temperature (hereinafter, referred to as an LD), by depositing GaN based compound semiconductor layers onto a sapphire substrate, and there is a growing interest in them. As light emitting devices with short wavelength such as described above, GaN based compound semiconductor is in the mainstream. Besides the GaN based compound semiconductor, the use of, for example, Groups II-VI compound semiconductor such as ZnSe/ZnMgSSe based or ZnO based compound semiconductor has been studied.
A ZnO based oxide has been conventionally utilized only in the form of amorphous and polycrystalline. In recent years, however, plasma technology for making a gas such as nitrogen into plasma has been advanced. Accordingly, the thin film crystal growth technology where the above-mentioned plasma technology is applied using a radical source molecular beam epitaxy (RS-MBE) apparatus has made significant advancement. Moreover, studies on ZnO single crystal have been proceeded by employing PLD and a vapor phase transporting method, and have reached to the level at which the laser oscillation can be observed by a laser beam excitation (see Solid Stat. Commun., by P. Yu et al., vol. 103, No. 8 issue, on pages 459 to 466, in 1977, or Appl. Phys. Lett., by D. M. Bagnall et al, vol. 170, No. 17 issue, on pages 2230 to 2232, in 1977).
As described above, studies on ZnO based oxide semiconductor have made advancement. However, ZnO based oxide semiconductor has disadvantage in that, as compared with GaN, ZnO has high concentration of residual carriers, and the value thereof reaches to 1018 cmxe2x88x923 of n-type. The simplest method for lowering the concentration of the residual carriers is to elevate the high growth temperature. However, the present inventors have confirmed that, if simply elevating the growth temperature on the substrate, Zn which has reached on the substrate revaporizes from the surface of the substrate at high rate, and the growth does not proceed. This is because the deposition coefficient of ZnO onto the sapphire substrate having the different characteristics from ZnO is small, and in the case of ZnO, the vapor pressure of Zn is high (for example, see The 8th SiC and related wide gap semiconductor workshop P-83, or The 47th Applied Physics related association party, articles for lecture, 29a-YL-3). For the above reason, the growth temperature on the substrate is set to as low as at approximately 300xc2x0 C. in many cases. However, at a low growth temperature, the concentration of the residual carries cannot be lowered, and the low concentration of the residual carriers, which is an essential condition for a semiconductor material, is not be attained.
Thus, it is considerable that a ZnO based oxide layer is once grown as a buffer layer at a low temperature, and after that, the temperature is elevated to high value at which an predetermined ZnO based oxide semiconductor layer such as a light emitting layer forming portion is grown. However, even in this method, the flatness of the buffer layer is degraded during elevating the temperature on the substrate. In this case, the ZnO based oxide semiconductor layer which constitutes the light emitting layer forming section has poor flatness accordingly, making it impossible to obtain improved light emitting characteristics.
The present invention has been made to solve the problems such as described above, and an object thereof is to provide a method for growing a ZnO based oxide semiconductor layer with excellent crystallinity and low concentration of residual carriers.
Another object of the present invention is to provide a method for manufacturing a semiconductor light emitting device with excellent light emitting characteristics such as a light emitting diode with excellent light emitting efficiency and a laser diode with low threshold current value by growing a ZnO based oxide semiconductor layer with excellent crystallinity.
The prevent inventors have conducted intensive studies to attain the growth of ZnO based oxide semiconductor layer without crystal defects, and with low concentration of residual carries. As a result of the studies, it was found out that the crystal defects and high concentration of the residual carries are resulted from the roughness on a surface of the growing ZnO based oxide semiconductor layer.
Specifically, as described above, if it is attempted to grow ZnO directly on a sapphire substrate at high temperature, ZnO is not sufficiently deposited onto the surface of the sapphire substrate, because the sapphire substrate has the different characteristics from those of ZnO. In addition, the vapor pressure of Zn is high. As a result, the growth of ZnO onto the sapphire substrate does not proceed, and the surface of the substrate becomes rough. The ZnO based oxide semiconductor layer which is grown thereon also has a rough surface resulted from the roughness of the surface of the substrate, and accordingly, has poor crystallinity. Even if the buffer layer is formed at a temperature as low as approximately 300xc2x0 C., and then, the temperature is elevated to approximately 600xc2x0 C. and an predetermined ZnO based oxide semiconductor layer is grown, thus-formed ZnO based oxide semiconductor layer still has poor flatness. It is also impossible to obtain a semiconductor layer with excellent crystallinity.
The present inventors have made intensive studies on the reason why excellent flatness cannot be attained even if a buffer layer is formed at a low temperature, and then, the temperature of the substrate is elevated to high temperature and an predetermined ZnO based oxide semiconductor layer is grown thereon. As a result of the studies, it was found the following. At the time when the temperature of the substrate is elevated to high temperature after the buffer layer is deposited, if the temperature is elevated in the state of supplying oxygen as is the case of a conventional material gas of Group V elements such as GaAs and GaN, etching effect occurs by the supplied oxygen during the elevation of the temperature. As a result, the flatness of the surface of the buffer layer is degraded, and the roughness is formed thereon.
Due to the roughness on the surface of the buffer layer, an predetermined ZnO based oxide semiconductor layer which is grown thereon at high temperature also has roughness because of the roughness of the buffer layer, and has poor crystallinity. To overcome this problem, the present inventors have reached the following finding. In the temperature elevating step, the supply of oxygen is stopped to eliminate the influence of oxygen. Then, the temperature of the substrate is elevated to a predetermined temperature and a necessary raw materials such as plasma oxygen and Zn is irradiated onto the buffer layer. In this manner, a semiconductor layer with excellent crystallinity can be obtained.
According to the present invention, a method for growing a ZnO based semiconductor layer includes the steps of: supplying raw materials containing oxygen onto a substrate at a temperature lower than a temperature at which a function layer made of a predetermined ZnO based oxide semiconductor is grown, so as to grow a buffer layer made of ZnO based oxide semiconductor; stopping the irradiation of the oxygen plasma and then elevating the temperature of the substrate to the temperature at which the function layer is grown; growing the function layer by supplying raw materials containing oxygen.
Here, ZnO based compound semiconductor means an oxide containing Zn. Specific examples thereof include ZnO, oxides of Group IIA and Zn, oxides of Group IIB and Zn, or oxides of Group IIA and Group IIB and Zn.
By use of the method of the present invention, a ZnO based oxide semiconductor layer is grown as a buffer layer at low temperature. In this manner, whereas the buffer layer itself has high concentration of residual carriers, this semiconductor layer has highly flat surface. Then, a predetermined ZnO based oxide semiconductor layer is grown thereon. Since the influence of oxygen is eliminated at the time when the temperature of the substrate is elevated, the temperature of the substrate can be elevated to at approximately 600xc2x0 C. while flatness of the surface of the substrate is maintained. In this state, since the raw materials such as oxygen and Zn are supplied to the surface of the substrate, the predetermined ZnO based oxide semiconductor layer is grown on the flat buffer layer. As a result, the ZnO based oxide layer growing on the flat surface grows along the flat surface and crystals of the buffer layer, thereby obtaining a ZnO based oxide semiconductor layer with very little crystal defects.
In particular, by growing the buffer layer under the condition of Zn rich, it is obtained to flatten the surface of the buffer layer and unitize polarity. Accordingly, the crystallity of the ZnO based oxide semiconductor layer growing thereon at high temperature has further improved.
Here, the condition where Zn is rich has the following meaning. In the relationship between the growth rate (shown by a longitudinal axis) and the supply amount of Zn (shown by a horizontal axis) shown in FIG. 3 for example, the supply amount of Zn (shown by a horizontal axis) is changed in the state where the supply amount of oxygen (O) is kept at constant value. The condition where Zn is rich is the growth condition where the supply amount of Zn is larger than the shoulder portion of the graph of FIG. 3 (the portion where the growth rate is saturated), and shows the relationship between the supply amount and the growth rate on the surface of the substrate at the time of growth.
In the relationship between the growth rate and the supply amount of Zn shown in FIG. 3, the shoulder portion of the graph shifts toward left side by lowering the growth temperature. Therefore, even if the supply amount of Zn is the same, by lowering the growth temperature, the condition where Zn is rich is easily attained. In the case of a MgZnO or CdZnO semiconductor layer where a portion of Zn is substituted by other elements such as Mg, the relationship means that the total supply amount of these elements is larger than the aforementioned portion where the growth rate is saturated.
A method for manufacturing a semiconductor light emitting device according to a specific example of the present invention includes the steps of; (a) growing a buffer layer made of ZnO based oxide semiconductor on the surface of the substrate at a temperature lower than the temperature at which a light emitting layer forming section made of a predetermined ZnO based oxide semiconductor is grown by employing MBE method, while irradiating raw materials containing oxygen radical on the surface of a substrate; (b) stopping the irradiation of the oxygen radical, and then, elevating the temperature of the substrate to the temperature at which a predetermined ZnO based oxide semiconductor layer constituting the light emitting layer forming saction is grown; and (c) sequentially growing semiconductor layers constituting the light emitting layer forming section, while irradiating the raw materials including the oxygen radical on the surface of said substrate.